In general, concrete, which is a composite composed of cement, sand, gravel, water, etc. mixed in suitable ratios, has a relatively high compression strength, but has very low tensile strength compared to compression strength and easily cracks. In order to reinforce concrete (to enhance tensile strength), rebars are bound (arranged) inside forms within which the concrete is poured.
These conventional rebar-concrete structures are widely used in various construction and civil engineering works because they have excellent compression and tensile strengths sufficient for use in building various types of structures and have excellent physical properties.
Rebars used in reinforced concrete are reinforcing bars and are divided into round rebars and deformed rebars. Round rebars have a smooth surface, while deformed rebars have sections and ribs.
Deformed rebars are more adhesive to concrete than round rebars and reduce crack width when concrete cracks. As main rebars, deformed rebars are generally used instead of round rebars. The modulus of elasticity of deformed rebars is 2,040,000 kg/cm2 and the coefficient of linear expansion is about 1.2×10-5/°C, which is almost the same as that of concrete.
These rebars are arranged in fabricating concrete structures for building construction and various civil engineering works. Since rebars are manufactured in standardized lengths, rebars of a predetermined length need to be interconnected in arranging the rebars.
Methods for connecting rebars during a typical rebar arrangement operation include a lap joint method, a welding joint method, a screw joint method, and a joining method with a joining device. In the lap joint method, wires are wound around ends of mutually overlapping deformed rebars to interconnect the same. With this method, rebar arrangement is relatively easily performed. However, this method is inconvenient since wires should be wound around each rebar. Moreover, the bending strength of the connecting portion is weak, which poses a safety problem. In the welding joint method, the end portions of the rebars are welded to each other, and the strength of the connection portion is good. However, the work is very inconvenient and takes a long time, causing construction delays. In the screw joint method, the connecting ends of deformed bars are screwed and interconnected. Thus, this method requires equipment for machining the connecting ends.
In view of the above, Korean Patent No. 1030579 (Apr. 21, 2011) discloses a reinforcing bar coupler. In this document, a pair of reinforcing bars is inserted into a hollow cylindrical tube from the outer ends of both sides of the hollow cylindrical tube and the reinforcing bars are arranged so as to be symmetrical about the center in the longitudinal direction, and the coupler includes a coupler cap provided with threads and a fastening spring coupled to the threads.
With this rebar coupler, each portion contacting the fastening spring is subjected to intense stress and the gripping portion is likely to be sheared.
Korean Patent No. 1003302 (Dec. 22, 2010) discloses a high strength steel reinforcement coupler. The disclosed reinforcement coupler includes a plurality of coupler housings having tapered portions and coupled to each other, and a plurality of coupler locks provided inside the coupler housings and having tapered portions corresponding to the respective tapered portions of the coupler housings.
In the conventional couplers configured as described above, the gripping portions of the rebars are supported by the respective coupler locks, and thus the supporting force is distributed over the gripping portions and displacement of the rebars is relatively large while the connected rebars are gripped.
In addition, manufacturing costs of the conventional couplers are relatively high. Further, there are many parts to tighten in using the coupler in the field, and thus it takes a lot of time to assemble the reinforcing bars.
Korean Patent Registration No. 0977658 (Aug. 17, 2010) discloses a bar connecting apparatus. The bar connecting apparatus includes a pair of bodies each having a cylindrical shape facing each other and adapted to receive an object such as a bar, a plurality of stoppers provided in the pair of bodies and having two or more steps to hold the object such as a bar tightly; a spring provided on a bottom surface of the plurality of stoppers to elastically support the plurality of stoppers in a longitudinal direction of the pair of bodies; and an elastic ring provided on the inner periphery of the plurality of stoppers to elastically support the plurality of stoppers in a radial direction of the pair of bodies.
The conventional bar connecting apparatus configured as described above has collet type stoppers stacked inside the body in the longitudinal direction, and accordingly tightening force is concentrated on parts corresponding to the respective collets. Particularly, as described above, since the body has a complex structure in which a plurality of stoppers is stacked in the longitudinal direction, it is difficult to manufacture the apparatus and rebars are interfered with by the stoppers in connecting the rebars to be coupled. In addition, since a plurality of stoppers is stacked in the longitudinal direction, displacement of the rebars becomes severe when the coupled rebars are tensioned as described above.